This invention relates to improvements in steering assemblies, and in particular but not exclusively to steering column assemblies for use in an electrical power assisted steering system for vehicles.
It is known to provide a steering column shaft which transmits torque from a steering wheel to a steering mechanism for turning the roadwheels of a vehicle or such like. For example, the steering mechanism may comprise a rack and pinion whereby the steering column shaft is provided on one end with a toothed wheel which engages a portion of the rack mechanism. Alternative arrangements including re-circulating ball steering mechanisms are also known.
In order to retain the steering column shaft in the desired position it is common practice to encase, at least partially, a portion of the shaft with a steering column support. The steering column is supported within the support to allow rotational movement, and may be located with bearings at one or more points along its length.
Because of the typical location of the steering wheel in front of a driver of a vehicle such as a car or lorry, there is a high risk of injury in a collision caused by impact of the driver upon the wheel. In a worst case, the wheel may be pushed back towards a driver at the same time as the driver is thrown towards, such as in a heavy impact causing high deceleration.
To reduce the risk of injury, it is known to make the steering column support housing collapsible by providing a portion of reduced strength which may deform under load. The steering column shaft itself may also be deformable.
A problem arises when the length of the steering column shaft and the support is insufficient to allow the provision of an adequate sized portion of reduced strength. This situation arises in the case of a vehicle fitted with an electrical power assisted steering system in which a motor is adapted to apply an assistance torque to the steering column shaft in response to a measurement of the torque applied by the driver. Torque is typically measured using a dedicated torque sensor and the location of the torque sensor on the steering column shaft and the need for a portion of the shaft over which the motor engages via a worm wheel or the like reduces the available length of steering column support which can be made deformable.
In accordance to a first aspect of the invention, a steering column assembly for a vehicle comprises first and second telescopic support parts defining a housing, an energy absorbing member, and a steering column shaft adapted to support a steering wheel at one end and substantially enclosed within the housing, at least one of the support parts being attached to a part of the vehicle and the other one of the support parts being supported in a normal position relative to the first part and adapted so that in an accident the second part moves telescopically relative to the first part against a force determined by the energy absorbing member acting between said first and second parts.
In one embodiment, the energy absorbing member may be deformed when the second part moves telescopically relative to the first part. The deformation may produce a reaction force which may resist relative movement between the first and second parts.
By the present invention, the need for a portion of reduced strength is eliminated. The first and second support parts may be of substantially rigid construction, and movement of the first and second support part relative to one another against the force of the energy absorbing member allows the housing to telescopically collapse in a controlled manner. Of course, a portion of reduced strength could be provided as well.
Preferably, the energy absorbing member comprises a compression spring which is located between the first support part and the second support part. The spring may comprise a flat wavy spring which may exert a radially directed force between the first support part and second support parts. This force generates friction between the spring and at least one of the first and second parts when they move telescopically relative to each other. The friction force can be adjusted by providing a more resilient wavy spring. Alternatively, using an identical spring but making the gap between the first and second support parts smaller will cause the wavy spring to be xe2x80x9cflattenedxe2x80x9d to a greater extent which will also increase the force. The spring need not deform when the two parts move relative to each other, and may be fixed relative to one of the parts.
At least one of the first and second support parts may each comprise an elongate U-shaped channel member having a base and a pair of side walls. The first part may be fixed relative to a part of the vehicle with the mouth of the channel facing downwards whilst the side walls of the second part may be adapted to engage the side walls of the first support part with the mouth of the second channel facing upwards. Of course, the two support parts could be arranged the other way. The two parts may be adapted to slide relative to one another by providing a groove in one part and a co-operating rail on the other part.
The first support part and the second support may be fixed to the vehicle through a mounting bracket. A frangible fastening means such as bolts having a weakened portion of plastics material may be used to fix the bracket to the vehicle which are adapted to break off under high loads. In an alternative, only the second part may be fixed to the vehicle through a mounting bracket with the second part at least partially supporting the first part.
The provision of the frangible fastening means provides additional safety as it allows the steering column assembly to break away from the vehicle. The mounting bracket may in turn be fastened to the vehicle through separate brackets which are also adapted to break in an accident.
The steering column shaft may also be telescopic, comprising at least a first and second portion adapted to move axially relative to one another at least when the first and second support parts move relative to one another.
Means may be provided for allowing the height of the steering column assembly to be adjusted. This may comprise a lever and cam mechanism for allowing relative movement between the first and second support parts and the mounting bracket.
A steering lock assembly may be provided on a portion of the first part. The lock assembly may comprise a locking portion adapted to engage with a part of the steering column shaft to substantially prevent rotation of the steering columns relative to the support part(s). It may be adapted to freely slide past the second part when the first part telescopes relative to the second part.
The steering column may further be adapted to allow movement of the steering wheel relative to the driver by lengthening or shortening the steering column assembly. It is envisaged that this could be achieved by making at least one of the first and second supports parts from two subsections which can move telescopically relative to one another. A locking lever may be provided which allows the driver to adjust the steering wheel when the locking lever is in its unlocked position but fixes the steering column assembly position when in its locked position.
The first one of the first support part and second support parts may be provided with one or more rails which are adapted to slide within one or more guides provided in the other one of the first and second support parts. Preferably, one of the support parts is provided with a first and second guide rail arranged in parallel. Each of the first and second guide rails may be adapted to co-operate with a corresponding guide channel in the other support parts. The first one of the guide rails and the corresponding guide channel may comprise interengaging co-operating non-planar location surfaces (suitably Vee shaped), whilst the second guide rail and its corresponding guide channel may comprise planar co-operating surfaces to provide free play between the second rail and its channel. Thus, the first guide rail and channel provide accurate location of the first part relative to the second part whilst the free play between the second rail and channel ensure that tolerances in the relative spacing between the two rails (or the two channels) is not critical. If no free-play is provided, the spacing between the rails and/or channels would need to be tightly controlled.
Preferably, plastic or other material may be injected into at least part of the space between the first and second parts of the steering shaft to take up any free play between the two parts. The material may be adapted to readily break when the first part is moved relative to the second part. In an alternate, a mechanical spring means may be provided.